[ma0641]

I have listened to this sound many times and after the bangs still believe I can hear something spooling down, a pump or a motor. You can get some duct support webbing and cradle the expansion tank, that's what they did to mine.

[hkstroud]

Suggestion (we are getting desperate).

Place your recorder next to the PRV (pressure reducing valve) and see what sounds you record.

[massplumber2008]

Could also use some perforated metal strap hanging (sold in 10 FT rolls at all home improvement stores) and crisscross under the expansion tank back up to the joists to support that tank better.

[Mike45plus]

A detailed log of the event including: date, time of day / night, weather, indoor / outdoor temps, duration, frequency, # of occupants, measures taken, special activities, etc., may help your investigation. Disconnect any seldom used, or unnecessary devices / appliances. Water piping, DWV piping, electrical conduit, and, metal vent & duct systems are the most common ways to transmit sound throughout a structure - make sure all fixtures, fittings, and appliances that use a solenoid, swing or spring check valve, mixing valve, tempering valve, pressure reducing valve, ballcock, isolation valve, auto fill valve, backwash connection, relief valve, back flow device, trap primer, venturi, etc, are thoroughly inspected & tested.
Inspect the chimney and vent connectors; make sure any barometric dampers ( sometimes located near the chimney top ) are working correctly. Have your gas supplier test the meter, relief valve, inlet & outlet pressures, gas valves & sequencing, pilots, main burners, flue baffles, and have them clock the meter dial for accuracy.
Shut the water main valve at night, this may reveal a clue - you can record the meter dial reading before and after. Someone mentioned installing gauges upstream & down from the meter - another way to monitor usage. I would also isolate each bathroom group - use one bathroom at a time for a two week cycle. Inspect & test all mechanical systems including: heating, cooling, generator, water conditioning / softener equipment, condensers, remote refridgeration, compressors, ice makers, wine coolers, float switch, storage tanks, expansion tanks, water hammer arrestors, electronic air cleaners, dampers ( manual & auto ), zone valves, booster pump, water pump, pressure switches, sump pumps, condensate pumps, humidifyers, dehumidifyers, ejector system, sewage pump, water fountain coolers, sprinklers, irrigation, circulators, venturis, relays / controls, pool filters & pumps, chemical injectors, exhaust fans, ventilation ( bathroom, range hood, combustion air, attic, whole house, make - up air, ), registers, grilles, steam generator, neutralizer...............................

To summarize : define / identify all possible contributors, and, eliminate them one at a time in an orderly fashion documenting each action taken......

[Mike45plus]

I apologize for the double post - the site was continuously redirecting me.....but, while I'm here, a few words about expansion tanks - no manufacturer recommends installing their tanks upside down, they all demonstrate the ideal location & orientation in their installation diagrams. Potable water expansion tanks may work upside down, but as Mass plumber has noted, they are very difficult to service & maintain, and, an incorrect install may void the warranty - which could be important if there is water / flood damage from a failed tank. Closed system, iron / steel expansion tanks should NEVER be installed upside down; air can get trapped in an upside down tank and corrode the exposed metal.................

[whatsthissound]

I apologize for the double post - the site was continuously redirecting me.....but, while I'm here, a few words about expansion tanks - no manufacturer recommends installing their tanks upside down, they all demonstrate the ideal location & orientation in their installation diagrams. Potable water expansion tanks may work upside down, but as Mass plumber has noted, they are very difficult to service & maintain, and, an incorrect install may void the warranty - which could be important if there is water / flood damage from a failed tank. Closed system, iron / steel expansion tanks should NEVER be installed upside down; air can get trapped in an upside down tank and corrode the exposed metal.................

Thanks for the extensive list of things to test in the previous post. We have kept track of dates and times:

It only lasts for as long as the recordings show (seconds) and happens no matter how many people are home, even when house is empty, and happens whether water is running or not.

As for the expansion tank mounting, a photo of the setup (the same from this thread) was sent to Amtrol and they said it was "Not a bad install, but the tank needs to be supported". When comments here conflict with the manufacturer, that just makes for more worry! They just might get another email regarding your comment.

We're also now monitoring water pressure with a lazy hand pressure gauge as suggested. So far, pressure on the house side of the PRV has stayed between 45 and 53 psi - EXCEPT for one spike to 117 psi. No banging sound inside the house since 8/3, no banging sound before the 117 reading (though there was a loud explosion sound then outside, from the direction of where some construction was going on). So far the gauge has only been monitoring for 2 days, and we're waiting to see what it reads when the next bang happens. It's strange actually hoping that it happens now, in order to get data.

[Mike45plus]

Whatsthis,
Amtrol has a box highlighted in their expansion tank installation instructions with the word " warning " written in it; immediately adjacent to the warning message, this statement is written: '' mount the tank vertically only ''. A diagram then shows the tank mounted vertically and connected below a horizontal pipe. Amtrol also says to be sure the piping is capable of supporting the tank in a waterlogged state. A tank mounted upside down has the potential to do some serious damage if it becomes waterlogged and comes crashing down; a flood is one thing, but bodily harm / injury makes someone liable - that is why we always follow manufacturers recommendations.
A pressure spike of 117 psi is EXTREME, dangerous, and, a potential contributor to the banging issue. Make sure the PRV is working correctly.
I like your spread sheet approach, perhaps you could transfer the data to a graph or chart where it will be easier to see any developing trends.

[hkstroud]

I would have remove that expansion tank and capped the pipe a long time ago. If for no other reason than just to see if anything different happens.

You don't need the expansion tank unless the PRV is also a check valve (if it is it should be shown on tag). You didn't have an expansion tank before this water heater install did you?

[whatsthissound]

No, there was no expansion tank prior to this water heater install (and the old water heater was 25 years old). The plumber said the expansion tank was required, and a Google search reveals that the county requires an expansion tank on a new water heater install.

The house was built ~37 years ago, so the PRV is probably that old.

The PRV tag is here:

[hkstroud]

The house was built ~37 years ago, so the PRV is probably that old.

I can assure you that the PRV is not that old. No indication of a check valve. Your own personal experience confirms that also.

The plumber said the expansion tank was required

Yes, that is what the plumber will tell you. But unless the local authorities have added that requirement to the latest national code they have adopted it, is just not true.

Note that I have had this same discussion with someone else in the last month or two. I have verified my position with the local county plumbing authority, which is as demanding (I believe) as any local authority in US.

As was related to me by the person I spoke with said, "Unless you have a PRV valve that incorporates a check valve, the entire county water system acts as you expansion tank."

One possible source of the noise could be the heat trap nipples if installed. I cannot tell form photos if you have heat nipples but I suspect so.
If noise persist after removing expansion tank, I would replace the cold water heat trap nipple.

[whatsthissound]

I can assure you that the PRV is not that old. No indication of a check valve. Your own personal experience confirms that also.

Owned the house 95% of that time and do not recall ever putting a PRV in. Following the cold water line through the basement, there doesn't appear to be a check valve (nothing else on the line between water meter and water heater besides the PRV).

Yes, that is what the plumber will tell you. But unless the local authorities have added that requirement to the latest national code they have adopted it, is just not true.

Note that I have had this same discussion with someone else in the last month or two. I have verified my position with the local county plumbing authority, which is as demanding (I believe) as any local authority in US.

As was related to me by the person I spoke with said, "Unless you have a PRV valve that incorporates a check valve, the entire county water system acts as you expansion tank."

Looking at the county code right now and it explicitly mandates an expansion tank on new and replacement water heater installs. It is an amendment to the Uniform Plumbing Code.

Also, regarding Mike45plus's concern about the orientation of the expansion tank, the code also says the tank can be installed vertical up OR vertical down (or horizontal, with support), per manufacturer's recommendation.

In 2013, Amtrol said:
www.WaterHeaterExplosions.com - Another great official website from Michael Leavitt & Co Inspections, Inc.
(they did not specify whether vertical up/down is better than the other)

One possible source of the noise could be the heat trap nipples if installed. I cannot tell form photos if you have heat nipples but I suspect so.
If noise persist after removing expansion tank, I would replace the cold water heat trap nipple.

Thanks, we'll add that to the list of things to check!